1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer and a facsimile machine and, in particular, to an image forming apparatus having a function of transferring an image formed on an image bearing member to a recording material and thereafter fixing the image thereon.
2. Related Art
An electrophotographic image forming apparatus is an example of such an image forming apparatus. With the electrophotographic image forming apparatus, an unfixed toner image corresponding to target image information is formed and borne on an image bearing member such as an electrophotographic photosensitive body by an image forming process using a heat-fixing visualizing agent (toner). The toner image is transferred by a transferring apparatus from the image bearing member onto a recording material, and the recording material having undergone the transfer of the toner image is introduced into a heating apparatus (heat-fixing apparatus), whereby the toner image is subject to a heat-fixing treatment as a permanently fixed image to be discharged as an image formed object (copy, print).
As the transferring apparatus used in such an image forming apparatus, a transferring apparatus of a noncontacting electrostatic transfer method is often used according to the recent tendency to eliminate ozone. The transferring apparatus of the noncontacting electrostatic transfer method is for applying a predetermined transfer bias from a power source to a transfer member such as a transfer roller located on a back side of a recording material, thereby attracting a toner image from an image bearing member with an electric force and transferring it onto the recording material.
As a heating apparatus for heat-fixing a toner image on a recording material, a heating apparatus is generally used which brings a pressurizing roller having elasticity in pressed contact with a rotational heating member having a built-in heat source such as a fixing roller or a fixing film and introduces a recording material to its press-contacting nip portion to perform a fixing operation of the toner image.
A pressurizing roller as a pressurizing member used in the above-mentioned heating apparatus is often provided with a heat-resistant elastic body such as silicone rubber on a core metal, which is supporting body and has rigidity and further, provided with a fluorocarbon resin layer, if necessary. The heat-resistant elastic body is required for its function as a part constituting the rotational heating member and the press-contacting nip portion. The surface layer may be provided for the purpose of improving a releasing property in order to avoid stains when adhesion and deposition of dirty toner or recording material components caused by offset or the like on the side of the rotational heating member are serious on the surface of the pressurizing roller.
However, a rubber material or a resin material, which is generally used as an elastic layer or a surface layer, is often an insulator. Thus, when dried paper with a high electric resistance as a recording material is passed through the pressurizing roller, the surface of the pressurizing roller is charged negatively due to friction with the paper. If toner held on the paper at this point is negatively charged toner, the toner repulses a triboelectrification potential of the surface of the pressurizing roller, resulting in a scattered image or an offset image.
A pressurizing roller is proposed in which a conductive material is dispersed in a rubber material or a resin material used for an elastic layer or a surface layer and treatment for lowering resistance is applied to the elastic layer or the surface layer in order to avoid such triboelectrification on the surface of the pressurizing roller. A resistance of one or more of these layers is reduced to 1013 xcexa9/sq or less in terms of a surface resistance or 1011 xcexa9cm or less in terms of a volume resistance. The layers with a reduced resistance are electrically grounded, whereby triboelectrification of the surface of the pressurizing roller can be prevented.
However, when an OHT (overhead transfer: a transparent recording sheet for an overhead projector) is used as a recording material to be passed through an apparatus provided with a pressurizing roller (conductive pressurizing roller) which has been subject to treatment for lowering resistance as described above, charges on the back of the OHT, which the pressurizing roller contacts, may flow to the ground to offset an image on the surface of the OHT.
In general, an OHT is made of a PET (polyethylene terephthalate) sheet coated with a surface-active agent or the like on its surface. Therefore, it has high insulating property in its thickness direction, but has a lower surface resistance. Due to such a condenser construction, if negatively charged toner is used, the OHT is subject to a positive transfer bias on its back when passing through a transfer portion. At this point, negative charges are induced on the surface of the OHT. In this state, the negative charges electrostatically balance with positive charges on the back of the OHT. However, if the OHT contacts a pressurizing roller, which has been subject to treatment for lowering resistance when passing through a fixing nip, the positive charges (transfer charges) on the entire back of the OHT flow to the ground. Thus, the front of the OHT is largely occupied by the negative charges and the negatively charged toner repulses the negative charges to have a weaker holding force, resulting in an offset image.
Further, when a recording material is plain paper, since it has lower insulating property in its thickness direction and a higher surface resistance in comparison with the OHT, such an offset image does not occur.
The present invention has been devised in view of the above-mentioned drawbacks, and it is an object of the present invention to provide an image forming apparatus that can suppress offset of an image.
It is another object of the present invention to provide an image forming apparatus that can suppress offset of an image regardless of a type of a recording material.
It is still another object of the present invention to provide an image forming apparatus that can suppress offset of an image even if a resin sheet is passed through fixing means provided with a pressurizing roller having a low resistance layer.
It is yet still another object of the present invention to provide an image forming apparatus including: an image bearing member; a transfer member for transferring an image from the image bearing member to a recording material; fixing means for fixing an image onto the recording material, the fixing means including a heating member and back-up roller for forming a nip cooperation with the heating member, in which the back-up roller has a conductive material containing layer; and in which a voltage applied to the transfer member when the recording material is a resin sheet is lower than a voltage applied thereto when the recording material is paper.
Other objects of the present invention will be more apparent from the following detailed description with reference to the accompanying drawings.